Typically, portable electronic equipment such as a notebook type personal computer is equipped with a battery (also called a secondary battery and storage battery) as a separate power supply dedicated for the equipment so that the equipment may operate as a single unit without being electrically connected to a receptacle of an external power source (commercial power supply) with an adaptor (AC adaptor). Moreover, in an environment in which an external power source is available, such as at a home, generally, such portable electronic equipment is not activated by a power supply having a limited power capacity, i.e. a battery, but is activated by being electrically connected to the external power source through an adaptor with the battery being attached to the electronic equipment so that alternating voltage acquired from the external power source is converted into direct-current voltage by an AC adaptor to obtain electric power.
For that purpose, electronic equipment of this type is provided with a charging apparatus which, when the equipment is linked to an external power source through an adaptor, charges a battery by the power of the external power source while the equipment is powered by the external power source. In a charging apparatus of this type, a problem may arise in that if an overcurrent flows from the adaptor side to the battery side, this overcurrent causes damage to the battery leading to battery degradation. Accordingly, it is contemplated that the current flowing to the battery side is fed back to the adaptor side to control the current flowing to the battery (for example, Japanese Patent Laid-Open No. 2002-78228). This arrangement reduces the tendency of an overcurrent to flow to the battery, thereby enabling to extend the battery life.
In this kind of electronic equipment, there may be a case in which the system load suddenly changes, such as when a function used theretofore is deactivated, for example, when a motor rotating a disk is stopped. In a typical battery, even if the responsiveness of the control loop of the battery side is increased (sped up), because of the fact that the response speed of the AC adaptor side may not follow it, the responsiveness of charging voltage during a sudden change of system load is likely to be such that an overshoot and undershoot take place in the charge voltage. Therefore, such responsiveness may cause an overvoltage to be supplied to the battery leading to a problem of battery degradation. Thus, for example, in lithium ion batteries, which are widely used as such battery in recent years, this kind of overshoot particularly has an influence on the battery degradation.